The relationship between inhibition of tension by Cd2+ in aorta and the kinetics of cadmium uptake and efflux was studied. Cd2+ (0.01-0.5 mM) inhibited the contraction of aorta to high-K+ (30 mM) in a dose-dependent manner. The high-K+ -induced tension completely returned to control values after 60 min washing with a solution containing 5 mM disodium edetate (EDTA) or 5 mM cysteine, following a treatment with 0.5 mM Cd2+ for 30 min; after washing with normal medium only 15% of the control response returned. Cadmium uptake increased with an increase of the Cd2+ concentration (0.01-0.5 mM). Aortae preincubated with 0.5 mM Cd2+ for 60 min were washed subsequently with a medium containing 5 mM EDTA or 5 mM cysteine. About 5% of the original tissue cadmium was retained after washing with EDTA or cysteine. It is suggested that Cd2+ binds chiefly to the surface membrane of aorta. It seems possible that the quantity bound is correlated with the degree of inhibition of tension.